Symmetric microlenses have been fabricated by means of a pulsed laser beam which, when focused at an end portion of a fiber, ablates progressively the circumferential periphery of the fiber lens. See U.S. Pat. No. 4,932,989, issued to H. M. Presby on Jun. 12, 1990, which is incorporated herein by reference. The teachings of Presby have been utilized in U.S. Pat. No. 5,011,254 issued to C. A. Edwards and H. M. Presby, which is incorporated herein by reference, in order to produce hyperbolically-shaped microlensed fibers. Greater than 90 percent coupling efficiencies have been achieved for coupling light from light sources, such as semiconductor lasers with symmetric modal outputs, to single-mode fibers and from the single-mode fibers to detectors by means of hyperbolically-shaped microlensed fibers fabricated using the teachings disclosed in C. A. Edwards and H. M. Presby.
Such microlenses afford relatively high coupling efficiency; however, this is useful only for lasers having a symmetric modal output. In such lasers the output beam profiles are circular or have ellipticity ratios close to 1:1, that is, the divergence of the output beam of the laser is substantially the same along an axis parallel and an axis perpendicular to the junction plane of the laser. However, many lasers have a highly elliptical beam shape emanating from the laser facet. For instance, in the use of erbium-doped fiber amplifiers which are pumped at a wavelength of 0.98 .mu.m, currently available pump laser diodes exhibit strong modal asymmetries, typically from 2.5:1 and up. There are also many high-power laser structures which are used to pump erbium-doped amplifiers at a wavelength of 1.48 .mu.m with non-circular outputs whose coupling could similarly be enhanced.
The use of symmetric microlenses utilized for coupling such lasers to fibers leads to a significant decrease in the coupling efficiencies. Such decrease is exemplified in FIG. 1 in which is plotted the coupling loss (in dB) versus the ratio of the x and y mode sizes. This plot arises by considering the coupling between an incident Gaussian laser beam, having different beam divergence angles parallel and perpendicular to the junction plane, with the circular mode of a single-mode fiber. It is seen from this plot that a beam asymmetry of, for example, 3:1 carries with it a loss penalty of more than 2 dB in coupling to the fiber.
Useful pump power coupled into a single-mode fiber is generally severely limited due to the difficulty of providing efficient coupling between the laser with a highly elliptical beam shape and the fiber. While hyperbolically shaped microlenses of Edwards and Presby, supra, may eliminate or at least reduce losses due to other factors, such as beam truncation and spherical aberration, an approximately 2 dB coupling loss still remains for elliptical beams having an eccentricity of about 3:1. Maximum coupling efficiencies that have been realized between such elliptical laser beams and the fibers using symmetric microlenses are only about 50 percent, with 25 to 35 percent being more typical. This means that, since about one-half of the laser output is not utilized, the laser has to be run at higher currents to yield the same coupled power into fiber that a more efficient coupling scheme could give. Running the laser at higher currents results in greater heat to be dissipated and raises questions of long term stability and reliability of the laser itself.
Attempts to increase coupling to elliptical beams with non-symmetric lenses have been reported in the form of externally mounted cylindrical lenses and a wedge-shaped fiber endface. See M. Saruwatari et al. "Semiconductor Laser to Single-Mode Fiber Coupler", Applied Optics, Vol. 18, No. 11, 1979, pages 1847-1856, and V. S. Shah et al. "Efficient Power Coupling from a 980 nm, Broad Area Laser to a Single-Mode Fiber Using a Wedge-Shaped Fiber Endface", J. Lightwave Technology, Vol. 8, No. 9, 1990, pages 1313-1318. In the latter case the wedge-shape approximates a cylindrical lens and a coupling efficiency of 47 percent was obtained.
Clearly, what is required for optimum coupling is an asymmetrical microlens which would transform the elliptical laser output to match mode profile of the circular single-mode fiber. Therefore, it is desirable to be able to fabricate asymmetric microlenses in an efficient and a controllably reproducible manner.